The present invention relates to liquid crystal devices.
A key criterion of a substrate material for plastic-substrate liquid crystal devices is physical and chemical stability. In many polymers, and particularly in polyester, physical and chemical stability is greatly enhanced by stretching the polymer. For example, polyester which has been stretched in two directions is marketed commercially as MYLAR.TM., and is very widely used for applications, such as magnetic tape recording, which require great physical stability. Although it is highly desirable to be able to use the physical and chemical stability of stretched polymers, and in particular of stretched polyester, for liquid crystal devices, the stretching process unfortunately also induces birefringence. See F. D. Bloss, "An Introduction to the Methods of Optical Crystallography", Holt, Rinehart & Winston, New York 1961. In particular, two-dimensional stretching, as is done to make MYLAR, induces biaxial birefringence. In a twisted nematic liquid crystal device, such biaxial birefringence induces vivid color fringe effects, which are highly undesirable in a liquid crystal device.
In co-pending U.S. patent application Ser. No. 304,134, filed Sept. 21, 1981, abandoned in favor of continuation U.S. application Ser. No. 531,574 filed Sept. 12, 1983, the use of a polymer (preferably polyester), which has been stretched in one direction only, as a substrate for liquid crystal devices is claimed. Such one-dimensionally stretched polyester still has birefringence, but the birefringence in uniaxial rather than biaxial. Color fringing caused by uniaxial birefringent substrates is easier to avoid, and one-dimensionally stretched polyester still has greatly improved physical and chemical stability.
It is well known within the physics of birefringent films that the polarization of incident light can be transformed in the film, e.g. from linear to elliptical. The degree of transformation is determined by the product of the birefringence and the sample thickness divided by the wavelength of the light used. That is, for a given birefringent material, if a certain thickness will produce a null in transmission of incident red light, a slightly smaller thickness will produce a null in transmission of incident green light between crossed polarizers.
The problem in fabrication of liquid crystal devices having birefringent substrates is that obtrusive interference colors often appear in the finished device. One reason for these interference colors is that, when the upper and lower substrates have parallel principal axes, and a twisted nematic liquid crystal is between the two substrates, the optical retardation of the two substrates will approximately cancel. That is, while one substrate alone will typically induce an optical retardation of, e.g., 30 times pi, the difference in thickness in the two substrates is likely to be far smaller, and typically will correspond to optical retardation of only a few times pi at optical wavelengths if nominally equal substrate thicknesses are used. Typically the retardation is such that strong, saturated colors are produced at low angles to the substrate surface. The colors vary in hue with the thickness variations in the polymer substrates. This produces an unacceptable appearance to the display.
Thus, for a device with two birefringent substrates, the effective optical thickness will be small enough that low-order interference colors are produced, and these colors are exceedingly bright and noticeable. These colors are not noticed in a single film in isolation, because a single film, such as a seven mil thick piece of stretched polyester, is so thick that the interference nulls are a very high order, e.g. of order 30, and the interference colors produced by such higher-order nulls are not noticeable at all, since the many large-bandwidth nulls overlap.
This problem can be avoided by using isotropic rather than birefringent substrates, but use of birefringent substrates is preferable because of their chemical stability.
Thus it is an object of the present invention to provide a liquid crystal device using stretched polymer substrates which does not show apparent interference colors.
A further difficulty in the prior art of liquid crystal devices with birefringent substrates is that the interference colors seen are particularly sensitive to microscopic variations in the film thickness. That is, even a very good quality piece of optical plastic is very likely to have much random variation on a scale at least comparable to a wavelength of light, and such variation is itself sufficient to cause interference colors to appear.
Thus it is a further object of the invention to provide a liquid crystal device using birefringent substrates which does not require that optically flat substrates be used.
The present invention solves these problems very simply, by providing a device in which the two birefringent substrates have substantially unequal thicknesses. Thus, even when the thicknesses of the substrates are subtracted from each other, the net optical retardation is still large, and sufficient that the nulls induced are higher-order nulls.
According to the present invention there is provided: a liquid crystal device comprising: first and second substrates each comprising a birefringent material; first and second conductor layers, said conductor layers respectively being deposited on respective inner faces of said respective substrates; first and second alignment layers respectively deposited on said first and second conductor layers; perimeter seal means for sealing the perimeter of said first substrate to the perimeter of said second substrate; and a liquid crystal material filling the cavity defined between said respective inner surfaces of said respective substrates; wherein said first and second substrates have significantly unequal thicknesses.